1. Field of the Invention
The present invention pertains to the field of flow metering technology including a system and method for use in measuring production volumes including a multiphase mixture of discrete phases, e.g., a mixture including oil, gas, and water phases. More specifically, the system and method determine a density of the oil in the multiphase mixture to more efficiently measure a flow rate of the oil.
2. Statement of the Problem
It is often the case that a fluid flowing through a tubular member contains a plurality of phases, i.e., the fluid is a multiphase fluid. As used herein, the term “phase” refers to a type of fluid that may exist in contact with other fluids, e.g., a mixture of oil and water includes a discrete oil phase and a discrete water phase. Similarly, a mixture of oil, gas, and water includes a discrete gas phase and a discrete liquid phase with the liquid phase including an oil phase and a water phase. The term “fluid” is used herein in the context that fluid includes gas and liquids.
Special problems arise when one uses a flowmeter to measure volumetric or mass flow rates in the combined multiphase flow stream. Specifically, the flowmeter is designed to provide a direct measurement of the combined flow stream, but this measurement cannot be directly resolved into individual measurements of the respective phases. This problem is particularly acute in the petroleum industry where producing oil and gas wells provide a multiphase flow stream including unprocessed oil, gas, and saltwater. Commercial markets exist only for the hydrocarbon products.
It is a common practice in the petroleum industry to install equipment that is used to separate respective oil, gas, and water phases of flow from oil and gas wells. The producing wells in a field or a portion of a field often share a production facility for this purpose, including a main production separator, a well test separator, pipeline transportation access, saltwater disposal wells, and safety control features. Proper management of producing oil or gas fields demands knowledge of the respective volumes of oil, gas and water that are produced from the fields and individual wells in the fields. This knowledge is used to improve the producing efficiency of the field, as well as in allocating ownership of revenues from commercial sales of bulk production.
Early installations of separation equipment have included the installation of large and bulky vessel-type separation devices. These devices have a horizontal or vertical oblong pressure vessel together with internal valve and weir assemblies. Industry terminology refers to a ‘two-phase’ separator as one that is used to separate a gas phase from a liquid phase including oil and water. The use of a two phase separator does not permit direct volumetric measurements to be obtained from segregated oil and water components under actual producing conditions because the combined oil and water fractions are, in practice, not broken out from the combined liquid stream. A ‘three-phase’ separator is used to separate the gas from the liquid phases and also separates the liquid phase into oil and water phases. As compared to two-phase separators, three-phase separators require additional valve and weir assemblies, and typically have larger volumes to permit longer residence times of produced materials for gravity separation of the production materials into their respective oil, gas, and water components.
Older pressure vessel separators are bulky and occupy a relatively large surface area. This surface area is very limited and quite expensive to provide in certain installations including offshore production platforms and subsea completion templates. Some development efforts have attempted to provide multiphase measurement capabilities in compact packages for use in locations where surface area is limited. These packages typically require the use of nuclear technology to obtain multiphase flow measurements.
Coriolis flowmeters are mass flowmeters that can also be operated as vibrating tube densitometers. The density of each phase may be used to convert the mass flow rate for a particular phase into a volumetric measurement. Numerous difficulties exist in using a Coriolis flowmeter to identify the respective mass percentages of oil, gas, and water in a total combined flow stream.
U.S. Pat. No. 5,029,482 teaches the use of empirically-derived correlations that are obtained by flowing combined gas and liquid flow streams having known mass percentages of the respective gas and liquid components through a Coriolis meter. The empirically-derived correlations are then used to calculate the percentage of gas and the percentage of liquid in a combined gas and liquid flow stream of unknown gas and liquid percentages based upon a direct Coriolis measurement of the total mass flow rate. The composition of the fluid mixture from the well can change with time based upon pressure, volume, and temperature phenomena as pressure in the reservoir depletes and, consequently, there is a continuing need to reverify the density value.
U.S. Pat. No. 4,773,257 teaches that a water fraction of a total oil and water flow stream may be calculated by adjusting the measured total mass flow rate for water content, and that the corresponding mass flow rates of the respective oil and water phases may be converted into volumetric values by dividing the mass flow rate for the respective phases by the density of the respective phases. The density of the respective phases must be determined from actual laboratory measurements. The '257 patent relies upon separation equipment to accomplish separation of gas from the total liquids, and this separation is assumed to be complete.
U.S. Pat. No. 5,654,502 describes a self-calibrating Coriolis flowmeter that uses a separator to obtain respective oil and water density measurements, as opposed to laboratory density measurements. The oil density measurements are corrected for water content, which is measured by a water cut monitor or probe. The '502 patent relies upon a separator to eliminate gas from the fluids traveling through the meter, and does not teach a mechanism for providing multiphase flow measurements when gas is part of the flow stream that is applied to the Coriolis flowmeter.
U.S. Pat. No. 5,535,532 describes multiple systems that measure the flow rates of oil, gas, and water. The '532 patent calculates the flow rate of oil based on a known or assumed value for the density of oil. One problem with the '532 patent is that none of the described systems calculate or measure the density of oil. The density of oil can be determined by taking a sample of the multiphase flow to a lab, which can be time consuming and expensive. The density of oil can also be assumed from previous data. However, the assumed density may not accurately represent the actual density of the oil.
Even three phase separation equipment does not necessarily provide complete separation of the oil phase from the water phase. Water cut probes are used to measure water content in the segregated oil phase because a residual water content of up to about ten percent typically remains in the visibly segregated oil component. The term ‘water cut’ is used to describe the water content of a multiphase mixture, and is most often applied to a ratio that represents a relationship between a volume of oil and a volume of water in an oil and water mixture. According to the most conventional usage of the term ‘water-cut’, well production fluids would have a 95% water-cut when water comprises 95 out of a total 100 barrels of oil and water liquids. The term ‘water-cut’ is sometimes also used to indicate a ratio of the total volume of oil produced to the total volume of water produced. A term ‘oil-cut’ could imply the oil volume divided by the combined oil and water volume. As defined herein, the term ‘water-cut’ encompasses any value that is mathematically equivalent to a value representing water or oil as a percentage of a total liquid mixture including water and oil.